Packet switches are at the core of modern data networks. When a packet is received by one of the ports of a switch, forwarding logic in the switch processes the packet header in order to determine the output port through which the packet should be transmitted. A switching core, such as crossbar switching logic, within the switch then transfers the packet to the output port. Even in high-speed switches, the header processing, forwarding computations, and packet queuing within the switch all add to the latency of data transfer across the network.
A number of solutions have been proposed and implemented to reduce the latency of patent switching. For example, U.S. Pat. No. 6,438,130, whose disclosure is incorporated herein by reference, describes a device for switching packets that includes a switching core and a plurality of ports, coupled to pass the packets from one to another through the switching core. One or more cache memories are respectively associated with one or more of the ports. Each of the cache memories is configured to hold a forwarding database cache for reference by the receiving port with which the cache memory is associated in determining the destination port of the packet. When a packet arrives at an input port of the switch, the port looks up the destination (e.g., MAC) address of the packet in its cache. When there is a cache hit, the switch sends the packet to the output port indicated in the cache. The port thus saves considerable processing time by avoiding having to read the port from the forwarding database of the switch.
As another example, the Cisco Nexus 3548 switch (produced by Cisco Systems Inc., San Jose, Calif.) offers a feature known as “Warp SPAN,” in which traffic arriving at one dedicated ingress port is replicated to a user-configurable group of egress ports. The packet replication happens without any filters or lookup mechanisms, before any traffic classification or ACL [access control list] processing occurs. Because traffic bypasses these processes, the latency for the replicated packets is said to be very low. The Warp SPAN functions independently of and simultaneously with normal traffic forwarding.